This invention relates to highway barriers that include guardrails extending along side a roadway, and to guardrails suitable for use in such a barrier.
Sicking U.S. Pat. No. 5,407,298 and Mak U.S. Pat. No. 5,547,309 disclose highway barriers including slotted guardrails. The guardrails are conventional steel beams having a W-shape in cross-section. Such a guardrail is well adapted to redirect an automobile after it has left a roadway, but it can provide excessive rigidity against column loads. Such excessive rigidity can result in a tendency of the guardrail to spear an axially impacting vehicle. In order to reduce this tendency, the guardrail disclosed in the Sicking '298 and Mak '309 patents includes longitudinally extending slots that reduce the maximum column load that can be supported by the guardrail.
Mak U.S. Pat. No. 5,503,495 discloses a guardrail cable release mechanism designed for use with a breakaway support post. The cable release mechanism includes a plate that defines a parallel-sided notch and a V-shaped entrance to the notch. This plate is placed between the threaded nut at the end of the barrier cable and the first breakaway support post. When the breakaway support post is broken in an axial impact, the cable moves out of the notch and V-shaped opening to disengage from the release mechanism.
One potential problem associated with the longitudinally slotted guardrail of the Sicking '298 and Mak '309 patents is that the longitudinal slots separate the guardrail into four parallel ribbons. Because the metal plate between the slots is relatively long and thin, there is little directional control over the direction of bending of the guardrail at the longitudinal slots. This creates the possibility that during an impact the guardrail may fold to an elbow-shape that may be lifted to the height of the windows of the impacting vehicle. If this were to occur, there would be an increased danger that the guardrail might penetrate through the windows into the occupant compartment of the vehicle as it continues to interact with the guardrail installation.
A potential problem associated with the guardrail cable release mechanism of the Mak '495 patent is that the slotted bearing plate might not release as quickly as desired when the breakaway support post is broken in an axial impact, particularly if the support post were to break at an oblique angle to the horizontal.